ViroPharma Inc.
VPHM
plans to resume clinical testing of a new formulation of its genetic-disease treatment Cinryze, combined with a drug-delivery technology supplied by Halozyme Therapeutics Inc.
HALO, +0.91%
after U.S. regulators investigated a potential safety issue.

The companies said Friday that the U.S. Food and Drug Administration, which had placed studies on temporary hold more than a month ago, believes the potential safety issue is limited to another company's drug-development program.

Shares of Halozyme shot up $1.73, or 27%, to $8.14, while ViroPharma rose $2.30, or 8.2%, to $30.48 in recent trading Friday.

Cinryze is currently approved to be administered by intravenous infusion to treat hereditary angioedema, a type of swelling that can afflict airways. It is a rare but serious genetic disease.

ViroPharma had been studying a new formulation of Cinryze that is injected subcutaneously, or under the skin, in combination with an enzyme developed by Halozyme, known as "rHuPH20." The enzyme is designed to facilitate the dispersion and absorption of the drug.

Halozyme also has partnerships with other companies that are studying drugs in combination with the enzyme. Baxter International Inc.
BAX, +0.17%
has sought regulatory approval for a combination of a drug dubbed HyQ with Halozyme's enzyme for the treatment of patients with immune-system deficiencies.

In August, Baxter said the FDA declined to approve the proposed combination because it wanted more information. The FDA raised concerns that some patients who received the proposed combination developed non-neutralizing antibodies against the Halozyme enzyme, and these potentially could affect reproduction, development and fertility, according to Baxter and Halozyme.

At the same time, the FDA placed on temporary clinical hold studies combining Cinryze with the Halozyme enzyme, as it evaluated the potential safety concerns in the Baxter studies.

On Friday, ViroPharma and Cinryze said the FDA informed ViroPharma the agency believes "the potential safety signals regarding antibodies to rHuPH20 that were detected in the clinical development program of another company's product are limited to that specific program."

Halozyme said the detected antibodies were "non-neutralizing" and not associated with any clinical adverse events.

ViroPharma said the FDA has advised it to amend the study protocol, allowing for increased laboratory sampling to monitor rHuPH20 antibody levels.

A Baxter spokesman said the company is working to address the FDA's request for additional information about its HyQ drug application.

An FDA spokeswoman said the agency can't comment on clinical studies of unapproved drugs or proposed new indications for drugs.

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